This invention relates to controlling monomer loss in the manufacture of fiber-thermoplastic matrix so that greater than 25% polymer by weight of the matrix is formed in the matrix while polymerization time is reduced to less than eight minutes. More specifically, monomer loss from a fibrous web of less than 0.25-inch thickness, impregnated with a polymerizable composition and subjected to in situ bulk polymerization is controlled by applying careful pressure and temperature controls during the polymerization.
Fiber-plastic combinations are well known. Resins have been used to enhance the wet strength of paper and as binding agents in cellulosic sound-insulating and filling materials. Patents that show a moldable fiber-thermoplastic matrix for use in decorative lamnnates, luggage shells, and corrugated material are U.S. Pat. Nos. 3,119,731, 3,121,656, 3,203,851; German Pat. No. 1,186,315; and Canadian Pat. No. 763,202. In these patents the cellulosic-thermoplastic fiber matrix is made by adding a finely divided thermoplastic polymer to the cellulosic fiber, usually into an aqueous suspension of the fiber. A fiber-thermoplastic polymer composition is obtained which is dried and subjected to heat and pressure to produce a densified composite article.
U.S. Pat. No. 3,232,824 discloses a process comprising: (1) addition of a polymerizable monomer or monomer/polymer emulsion to an aqueous suspension of cellulosic fiber; (2) emulsion polymerization of the monomer in the aqueous suspension; (3) removal of the water to produce a web; (4) drying; and (5) molding the dried material under heat and pressure. Since polymerization occurs in aqueous suspension, water must be removed from the fiber web and the web dried before the impregnated material can be molded and shaped by heat and pressure. Further, in emulsion polymerization, surfactants are usually required to stabilize the monomer or monomer/polymer emulsion, adding additional expense to the overall process. In addition, emulsion polymerization processes are quite time-consuming, usually taking hours to accomplish a useful amount of polymerization and polymer deposition.
Prior art showing production of fiber-polymer compositions by in situ polymerization is considerably more limited. Bulk polymerization of polymerizable monomer in a fiber matrix, that is, polymerization of substantially all of a reactant liquid consisting essentially of monomer or monomer/polymer without use of any emulsion system or medium, eliminates the drying and surfactant requirements but creates other problems. If vinyl monomers are polymerized in a fiber matrix, monomer losses from evaporation during polymerization can be prohibitively high due to the great volatility of such monomers. Further, the polymerization of vinyl monomers is generally an exothermic reaction once initiated, and without the "heat sink" capacity of an emulsion medium, the temperature and pressure conditions of bulk polymerization must be closely controlled to prevent runaway reactions which produce charred product and sometimes explosions. Close control is particularly critical when extremely rapid polymerization is sought by using high concentrations of heat activated polymerization initiators.
A non-emulsion polymerization of vinyl monomers in a cellulosic matrix is suggested in U.S. Pat. No. 3,083,118. Disclosed is a process of selectively polymerizing vinyl monomers within and/or upon host polymeric materials, such as wood fibers, having ion exchange capacity. The patentee begins with a host material having inherent ion exchange capacity, or chemically attaches an ion exchange material to the host, then chemically attaches a polymerization catalyst or initiator to the ion exchange material, and finally exposes the treated host material to a monomeric liquid which may be a monomeric solution, suspension, or emulsion to cause polymerization essentially only at sites on the host material of chemically bound catalyst or initiator. The process has many disadvantages. The amount of polymer deposition possible depends upon the ion exchange capacity of the host material. The process is slow (1) because of the need to chemically attach the catalyst or initiator to the host material, and to wash away excess catalyst or initiator before polymerization can be attempted, and (2) because it is possible to secure only a relatively small amount of initiator or catalyst to the host material due to its limited ion exchange capacity. Further, the patentee's prime concern, as suggested by each of his 181 illustrative examples, is in the areas of emulsion, suspension, or solution polymerization, which have inherent problems of their own as a result of the carrier liquids used in those processes.
In situ bulk polymerization of vinyl monomers saturating 1 inch wood blocks is shown in an article by Beall, Meyer and Skaar, "Direct RF Heat Curing of Wood Plastic Composites," 16 Forest Products Journal, No. 9 at 90 (September 1966). In Beall et al, aspen and basswood blocks were soaked in a polymerizable composition consisting of methyl methacrylate monomer and up to 1/2% by weight of the composition of a thermally activated free radical initiator such as benzoyl peroxide for 15-25 minutes. Polymerization was initiated by the heat provided by a constant temperature water bath regulated at 68.degree. C. A thermo-couple inserted in the block recorded the center temperature changes as the polymerization proceeded to completion from initiation. The temperature measurements were plotted as a temperature vs. time curve and show exothermic peaks during the polymerization at about 150.degree. C, well above the boiling point of the pure monomer at 100.degree. C. No attempt was made to control the temperature of the polymerization reaction. The only effort to control monomer loss was the wrapping of the saturted blocks in aluminum foil. It was noted by Beall that some losses of monomer occurred through evaporation and that there was a residual of uncured monomer in the blocks after process completion. Vacuum drying was found to be necessary to remove uncured monomer to change the polymer from the rubbery to the rigid state. The minimum time necessary to effect polymerization shown by Beall was about 70 minutes, when employing direct heating. Beall does show that an increase in peroxide concentration from 0.2 to 0.5% by weight of the polymerizable composition reduces the time to the exothermic peak. Beall does not anywhere suggest that monomer loss may be minimized by controlling temperatures, thereby allowing faster processing times.
Marks, in U.S. Pat. No. 2,516,064, describes a process for polymerization of a dimethacrylate ester of a glycol or mixture of glycols, with the Marks' invention lying in the use of a cobalt nitrate catalyst. While primarily directed to the manufacture of massive castings through bulk polymerization, Marks in passing notes that a product having a hard surface may be made by in situ polymerization of a monomer-coated methacrylate sheet. Marks, concerned with the large castings, does not address the problems of monomer loss and speed of polymerization that are inherent in handling a fast-moving continuous sheet. Marks indicates reaction temperatures of 0.degree. to 130.degree. C, and reaction times of greater than 55 minutes to complete polymerization.